The scanning schemes used by X-ray computed tomography apparatuses (X-ray CT apparatuses) are roughly classified into circular orbit scanning and helical scanning. The bed is not moved during circular orbit scanning. The bed is moved along the body axis of a subject during helical scanning.
A multi-array X-ray detector including a large number of X-ray detector arrays, e.g., 64 arrays, has recently been introduced. Introducing multi-array X-ray detectors has increased the occasion of scanning a wide range from a chest region to an abdominal region of a subject with one helical scan. In this case, in order to reduce image noise, the beam pitch is set to 1 or less, which is relatively slow.
In some cases, the lung fields are helically scanned with X-rays. The lungs are located near the heart, and always move accompanying the pulsation of the heart. When the beam pitch is set to 1 or less in helical scanning of the lung fields, projection data which can contribute to image reconstruction is acquired by an amount corresponding to one or more rotations of the X-ray tube. Using all these projection data for image reconstruction will generate high-quality image data with reduced image noise. In contrast to this, when projection data corresponding to one or more rotations is used for image reconstruction, the movement of the lungs accompanying heartbeats sometimes degrades the time resolution and makes the image less sharp.